Molecular Weight Dependence on the Disintegration of Spin-Assisted

May 29, 2013 - ABSTRACT: We present the effect of molecular weight. (MW) of polyelectrolytes (PEs) on the disintegration behavior of weak PE multilaye...
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Molecular Weight Dependence on the Disintegration of SpinAssisted Weak Polyelectrolyte Multilayer Films Yeongseon Jang,† Jooyeon Seo,† Bulent Akgun,‡,§,∥ Sushil Satija,‡ and Kookheon Char*,† †

The National Creative Research Initiative Center for Intelligent Hybrids, The WCU Program of Chemical Convergence for Energy & Environment, School of Chemical & Biological Engineering, Seoul National University, Seoul 151-744, Korea ‡ NIST Center for Neutron Research, NIST, Gaithersburg, Maryland 20899-6102, United States § Department of Materials Science and Engineering, University of Maryland, College Park, Maryland 20742, United States S Supporting Information *

ABSTRACT: We present the effect of molecular weight (MW) of polyelectrolytes (PEs) on the disintegration behavior of weak PE multilayer films consisting of linear poly(ethylene imine) (LPEI) and poly(methacrylic acid) (PMAA). The multilayer films prepared by the spin-assisted layer-by-layer deposition have well-ordered internal structures and also show the linear thickness growth behavior regardless of MWs of PMAA. The well-defined weak PE multilayer films were subject to disintegration into bulk solution when the electrostatic interactions between LPEI and PMAA layers were reduced by treatment at pH 2. However, we demonstrated the change in the disintegration mode and kinetics (i.e., from burst erosion to controlled surface erosion) as a function of MW of PMAA based on neutron reflectivity and quartz crystal microbalance with dissipation, revealing the correlation between the structural changes and the viscoelastic responses of the weak PE films upon pH treatment. Also, the unique swelling behavior as well as the significant increase in dissipation energy was monitored before the complete disintegration of the multilayer films containing high MW PMAA, which is believed to originate from their slow rearrangement kinetics within the film. We believe that the results shown in this study provide chain-level understanding as to the MW-dependence on pH-triggered disintegration mechanism of weak PE multilayer films.

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disintegration behavior of LbL multilayer films through thermal or chemical cross-linking,22 the insertion of diffusion barrier layers23 or blending with strongly binding materials24 into each bilayer. However, such post-treatment and the incorporation of strong binding materials could cause problems such as the decrease in ingredient activity or the interfacial disruption between internal layers.24 In this regard, the molecular weight (MW) of PE chains could be one of critical and intrinsic control parameters to tune the disintegration behavior of multilayer films without additional post-treatments or the insertion of other strongly binding molecules. It has been reported that the PE MW has a great effect on the surface characteristics and adsorption properties of the multilayer films25−28 as well as on the pH-triggered pore formation of multilayer films.29,30 However, to our knowledge, there are few reports dealing with the disintegration mechanism of multilayer films as a function of MW of PE chains incorporated,31 especially with quantitative analysis that provides both chain- and thin film structural-level of under-

INTRODUCTION The layer-by-layer (LbL) deposition is a powerful method to engineer functionalities of multilayer thin films by controlling intermolecular interactions between internal layers. Since the fundamental concept of the LbL assembly of oppositely charged polyelectrolytes (PEs) was introduced,1 many researchers have investigated the adsorption behavior of diverse polymer chains into multilayer films as a function of deposition parameters and types of intermolecular interactions.2−6 Consequently, many functional multilayer platforms based on the LbL assembly have been developed for extensive applications, as well documented in several review articles.5−12 In addition to the adsorption behavior of polymers into multilayer films, the release of active ingredients and/or the disintegration mechanism of multilayer films have recently received much attention to develop functional controlled release platforms.12−14 In particular, stimuli-triggered release platforms based on the LbL assembly have been realized as a function of external stimuli such as pH,15,16 temperature,17,18 or ionic strength19−21 to name a few, because the responses to such stimuli could increase functionalities and applications of the multilayer films. Furthermore, many research groups have made several attempts to control the stimuli-triggered © 2013 American Chemical Society

Received: April 15, 2013 Revised: May 15, 2013 Published: May 29, 2013 4580

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standing.32 In particular, since multilayer films based on weak PEs typically show rapid morphological change and subsequent disintegration when they are subject to low pH treatment,33,34 it is, in general, difficult to investigate the disintegration mechanism in detail when weak PE multilayer films containing short MW PEs start to disintegrate by pH triggering. Therefore, we believe that more detailed analysis on the internal structure of weak PE multilayered films upon external stimuli, such as swelling, rearrangements, and disintegration modes, is still lacking in the case of well-defined multilayers prepared by the spin-assisted LbL deposition based on the electrostatic interactions between weak PE pairs. Furthermore, revealing the correlations between structural changes and viscoelastic responses of the multilayer films as a function of PE MW could provide further insights on the controlled disintegration of designed multilayer films. Herein, we have studied the MW-dependent disintegration of weak PE multilayer films consisting of linear poly(ethylene imine) (LPEI) and poly(methacrylic acid) (PMAA), when the electrostatic interactions between LPEI and PMAA layers were diminished by post-treatment at low pH. We demonstrate that the controlled disintegration of the [LPEI/PMAA]n multilayer film is possible by controlling PMAA MW, based on neutron reflectivity (NR) measurements. Also, the corresponding dissipation energy of the multilayer films during the posttreatment was monitored by quartz crystal microbalance with dissipation (QCM-D). QCM-D is useful for monitoring the dynamics of adsorption or desorption of materials deposited on quartz crystal substrates and the changes in viscoelastic properties of adsorbed films in aqueous environment,35−37 allowing us to quantitatively analyze the in situ disintegration behavior of the multilayer films in the present study. By combining both NR and QCM-D measurements and analyses, the different disintegration modes of multilayer films were identified concomitantly with the changes in the energy dissipation within the films as a function of PE MW. The results reported in the present study would provide both chainand structural-level of understanding as to the effect of PE MWs on the pH-triggered disintegration behavior of weak PE multilayer films, giving insights for the development of functional controlled release platforms based on the spinassisted LbL deposition.

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[LPEI/PMAAMW]n (n: number of bilayers), were prepared on the piranha-treated hydrophilic substrates by the spin-assisted LbL deposition method with a spin-rate of 3000 rpm for 30 s for each deposition, followed by two consecutive washing spin steps to remove excess PEs with pH 5.0 DI water. To check the effect of LbL deposition method on the adsorption behavior of PMAAs with different MWs, we also prepared the multilayer films by the dipassisted LbL deposition: dipping into each PE solution for 15 min, followed by three consecutive washing steps for 2 min, 1 min, 1 min with gentle agitation as programmed in a Carl Zeiss DS50 slide stainer. Characterization and Post-Treatment of Weak PE Multilayer Films. Total film thicknesses in dried condition were measured by a variable-angle multiwavelength ellipsometer (Gaertner L2W15S830, Gaertner Scientific Corp.). The pH-triggered changes in the internal structure of the multilayered films were monitored by neutron reflectivity (NR) as described in our previous report.24 In order to secure enough neutron contrast as well as to facilitate the monitoring of the film internal structure based on NR, the multilayer films were constructed by replacing protonated PMAA layers with d-PMAA chains which have similar MW in every fourth bilayer deposition. These multilayer structures yield four peaks in the neutron scattering length density (SLD) profiles and all the NR data were fitted with the LbL box model, as also described in details in our previous paper.24 From the changes in the position, width, and amplitude of the peaks in the SLD profiles, we obtained the information on the disintegrated amount (%) and reduced thickness (%) of the post-treated films as compared to the as-casted films. To trigger the internal structural changes in the multilayer films, as-prepared films were immersed in pH 2.0 water for 10 min and then gently washed with same water used for the post-treatment, followed by relevant drying process under N2 stream. The degrees of ionization (%) of PMAA PEs incorporated into the multilayer films were characterized with FT-IR (FT-IR-200 spectrometer, JASCO Corp.). The multilayer films for FT-IR measurements were prepared on CaF2 crystals, which are transparent to IR light. Furthermore, the release and swelling kinetics based on the corresponding changes in viscoelastic properties of multilayer films were monitored by QCM-D and Q-Tools analysis. Quartz Crystal Microbalance with Dissipation Monitoring (QCM-D). The pH-triggered changes in both frequency (Δf n) and dissipation (ΔDn) of a multilayer film deposited on an Au sensor crystal (QSX301) were monitored by QCM-D (Q-Sense D300, QSense AB). In the QCM-D measurements, the changes in the resonance frequency and the dissipation factor are mainly related to mass uptake or loss at the surface of a quartz sensor and viscoelasticity of the film, respectively. The basic principle and measurement techniques of QCM-D measurements have been described in detail elsewhere.35−37,39,40 Every multilayer film was stabilized in pH 5.0 water, which is the same pH for the initial film deposition, for more than 100 min in order to set the baseline to monitor the pH-triggered changes in QCM frequency as well as dissipation energy in water setting. After stabilization, 0.8 mL of low pH water (i.e., pH 3.0, 2.5, 2.2, or 2.0) was injected to the sample chamber. The applied voltage is sequentially pulsed across the Au sensor crystal, allowing the shear wave to dissipate as well as the simultaneous measurements of absolute dissipation and absolute resonant frequency of the crystal for all four overtones (n = 1, 3, 5 and 7, i.e., 5, 15, 25, and 35 MHz). These overtones were used to characterize the viscoelastic properties of multilayer films adsorbed onto Au sensor crystals. All the measurements were taken at 25 ± 0.02 °C. Since Δf1 and ΔD1 were typically noisy due to insufficient energy trapping, frequency changes in the third overtone Δf 3/3 (Hz) were compared among the multilayer films with different MWs. Particularly, in order to compare the disintegration kinetics of multilayer films with different MWs, the Δf 3/3 (Hz) as a function of post-treatment time was converted into the relative percentage (%) of the final stabilized frequency after sufficient disintegration of films because the adsorbed film mass is slightly different in each sample.

EXPERIMENTAL SECTION

Materials.38 Linear poly(ethyleneimine) (LPEI, Mw = 25 000 g/ mol) and poly(methacrylic acid)s (PMAA, Mw = 15 000 g/mol and Mw = 100 000 g/mol) were purchased from Polysciences. PMAAs with different molecular weights (Mw = 32 400 g/mol (PDI = 1.08) and Mw = 226 000 g/mol (PDI = 1.08)) and deuterated PMAA (d-PMAA, Mw = 43 000 g/mol (PDI = 1.08) and Mw = 180 000 g/mol (PDI = 1.10)) were purchased from Polymer Source. All polymers were used as received. Silicon wafers and Au sensor crystals (QSX 301, Q-Sense) were used as substrates to prepare blend multilayer films for further characterization. Preparation of Weak Polyelectrolyte Multilayer Films. Weak polyelectrolytes (PEs), LPEI and PMAA, were chosen for cationic and anionic polymer solutions to prepare LbL multilayer films based on the electrostatic interactions. All the polymers were initially dissolved in 18 MΩ Milli-Q water with concentrations of 0.01 M based on repeat units and then the polymer solutions were adjusted to pH 5.0 by adding diluted HCl or NaOH solutions. All the substrates were treated with piranha solution, mixtures of 70 vol % H2SO4 and 30 vol % H2O2, for 20 min at room temperature, followed by thorough washing with DI water and drying under nitrogen stream. The multilayer films with different molecular weights (MWs) of PMAA, 4581

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On the other hand, in the dip-assisted LbL films, the MW of the PMAA chains has a significant effect on the growth of bilayer thickness and total film thickness because the PMAA chains have the different characteristic sizes in dilute solutions, along with the different conformations, depending on their MWs and charge densities.43 The PE chains of which diffusion is driven by the electrostatic interactions are adsorbed onto a substrate by almost maintaining their own conformation and characteristic size in dilute dipping solutions because any other external forces do not have an effect on the adsorption of the PE chains in contrast to the spin-assisted LbL deposition. Since the weak PE chains, LPEI and PMAA investigated in this study, were deposited at pH 5.0 close to their pKa values (i.e., 4.8−5.0 for LPEI44 and 5.5−6.0 for PMAA45), the partially charged weak PE chains typically take loop and tail conformations during the dipping-based LbL deposition. It is well-known that the polymer chains adopting more loopy conformations yield thicker layers.46 As a result, in the present study, every PMAA chain adopts the coiled conformation at deposition pH 5.0 with different characteristic size depending on the PE MW.47 Therefore, in the dip-assisted LbL deposition, the bilayer growth increases with the increase in MW of PMAA chains deposited within the multilayered film. From the comparison of multilayer growth behavior between the dip- and the spin-assisted LbL deposition methods, it is confirmed that the MW and conformation of weak PE chains employed for the LbL deposition significantly influences the interaction between polyanion and polycation layers, particularly in aqueous media. Therefore, in order to investigate the MW effect on the disintegration of LbL films, we prepared model weak PE multilayer films by the spin-assisted LbL deposition, showing the similar growth rate and conformations, independent of PE MW, in adsorbed state among samples (i.e., multilayered films prepared with different PE MWs). The different adsorption behavior with different PE MWs, as shown in the dip-based deposition, could be an additional parameter to be considered to interpret the disintegration behavior of multilayer films. In addition, the spin-assisted LbL films provide well-ordered internal structures, which, in turn, facilitate the monitoring of the changes in internal structure using NR, as well demonstrated in Figure 2. NR Studies on the pH-Triggered Changes in Internal Structures of Weak PE Multilayer Films with Different MWs. NR measurements have been performed to investigate the disintegration characteristics by monitoring the structural changes in the internal layers of weak PE multilayer films as a function of MWs. As shown in Figure 2, every spin-assisted [(LPEI25K/PMAAMW)3(LPEI25K/d-PMAAMW)1]4 multilayer film has a well-organized internal structure, which is confirmed by regularly spaced Bragg peaks in the NR data as well as welldefined four sharp peaks clearly evident in the SLD profiles. The alterations in the internal structures of these well-defined weak PE multilayer films were induced simply by changing the external solution pH. Recently,24 we have observed that the multilayer films with low MW, [(LPEI25K/PMAA15K)3(LPEI25K/d-PMAA43K)1]4, led to burst erosion at pH 2 due to the complete dissociation of electrostatic binding sites between LPEI and low MW PMAA, which have a small number of repeat units in a single chain. However, the multilayer films composed of higher MW (i.e., 32.4K, 100K, and 226K) PMAA chains demonstrate the maintained multilayer structures with different characteristics as a function of PMAA MW. In the case of the [(LPEI25K/

RESULTS AND DISCUSSION The model weak PE multilayer films, [LPEI25K/PMAAMW]n (n = number of bilayers), were composed of positively charged LPEI with MW of 25 000 g/mol and negatively charged PMAA with different weight average MWs of 15 000, 32 400, 100 000 and 226 000 g/mol. The detailed characteristics of PEs used in the present study are summarized in Table S1 in Supporting Information. Adsorption Behavior of Weak PE Multilayer Films with Different MWs. Weak PE LbL films show different bilayer thickness growth depending on the deposition methods. The multilayer films deposited by the spin-assisted LbL deposition demonstrated the similar growth rate for PMAA chains with different MWs (Figure 1A), while the ones created by the dip-assisted LbL deposition showed considerably different growth rate depending on MW of PMAA chains (Figure 1B).

Figure 1. Growth curves in the bilayer thickness of weak PE multilayer films, [LPEI25K/PMAAMW]n, with different MWs (○, 15K; ▽, 32.4K; □, 100K; ◊, 226K) of PMAA prepared by the (A) spin- and (B) dipassisted LbL deposition. Error bars denote ±1 standard deviations.

In the case of the spin-assisted LbL deposition, the adsorption of PE chains and the elimination of water molecules are almost simultaneously achieved in a short time as well as the external centrifugal and air shear force at a high spinning rate have an effect on the thin film formation.41 In particular, the air shear force driven by the spinning significantly enhances the surface planarization of the LbL films, and the PE chains are adsorbed onto oppositely charged surfaces with collapsed and tightly bound conformation regardless of their MWs. This more laterally stretched conformation causes the film to be thinner, as shown in Figure 1. The spin-assisted LbL films based on the electrostatic interactions have the persistent layers with the least internal mixing and smooth air surfaces when compared with the dip-assisted LbL films.42 4582

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Figure 2. NR curves with best fits in panel a and SLD profiles in panel b of weak PE multilayer films with (A) low MWs, [(LPEI25K/PMAA32.4K)3/ (LPEI25K/d-PMAA43K)1]4 and (B) high MWs, [(LPEI25K/PMAA100K)3/(LPEI25K/d-PMAA198K)1]4. Films initially deposited at pH 5 are represented by circle symbols in the reflectivity panels and by solid lines in the SLD panels while the films post-treated at pH 2 for 10 min are represented by square symbols in the reflectivity panels and by dotted lines in the SLD panels.

PMAA32.4K)3(LPEI25K/d-PMAA43K)1]4 multilayer film, the posttreated film loses approximately 30% of its original mass, mostly from the top surface, along with 38% reduction in film thickness without disrupting the well-ordered structure close to the substrate. This result indicates that protonated PMAA chains and loosely bound LPEI chains rapidly disintegrate and are released all together from the top film surface due to faster diffusion of the chains near air interface into bulk solution when compared with the chains buried deep into the film.48 It is also confirmed that the remaining layers on the substrate side are more mixed (Figure 2A) than as-prepared state, which is confirmed by the fact that the neutron SLD values for protonated layers are increased while the amplitude of the deuterated marker layers are decreased with broader interlayer roughness. On the other hand, the post-treated multilayer film with higher MW, [(LPEI25K/PMAA100K)3(LPEI25K/d-PMAA198K)1]4, almost maintained its internal structure and lost only about 3% of its original mass from the top surface (Figure 2B). The total film thickness after the post-treatment is reduced only 11% of its initial thickness. In addition, we noticed that the bottom layers were almost intact after the post-treatment while there is some mixing between protonated layers and the deuterated marker layer near the top surface when compared with the asprepared film. Overall, it seems that the internal rearrangements of PE chains are more dominant at the top film surface, during

the same post-treatment time of 10 min, when compared to the fast disintegration of multilayer films containing shorter PMAA chains (15K and 32.4K). The difference in the film profile after the post-treatment could be explained by the slower desorption and dissolution kinetics of high MW PE chains. Since the initial number of electrostatic binding sites per single PMAA chain interacting with LPEI chains is believed to be the predominant factor controlling the film disintegration kinetics, the well-ordered multilayer film with higher PMAA MW demonstrates the preference toward maintaining its internal structures before it is fully disintegrated into bulk solution. From the maintenance of the Bragg peaks in the raw neutron reflectivity data and the SLD profile after posttreatment, we also safely confirm that the main skeleton of the [(LPEI25K/PMAA100K)3(LPEI25K/d-PMAA198K)1]4 multilayer film is almost protected with well-ordered internal layers even though some LPEI and PMAA chains disintegrate from top surface, as compared to [(LPEI25K/PMAA32.4K)3(LPEI25K/dPMAA43K)1]4 film post-treated at same condition. Based on the NR results, it is clearly demonstrated that the protonation of PMAA chains and corresponding charge misbalance which was triggered by post-treatment at pH 2 induce the disintegration of weak PE multilayer films from the top surface but the simple increase in PMAA MW of the multilayer films could transform from the burst erosion (i.e., immediate film disintegration) to the surface erosion (i.e., 4583

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for longer time before the complete disintegration of the films when higher MW PMAA chains were incorporated into the multilayer films. The negative sign in frequency shifts which is clearly shown for the cases of the [LPEI25K/PMAA100K]16 and [LPEI25K/PMAA226K]16 multilayer films implies that the films gain some weight relative to the initial state deposited at pH 5.0. Such additional swelling, typically observed with high MW PE multilayer films, indicates that the multilayer films containing higher MW PEs tend to retain the well-ordered internal structure before full disintegration into bulk solution partly due to relatively high penalty for conformational change. Nevertheless, the masses of multilayer films containing high MW PMAA chains, [LPEI25K/PMAA100K]16 and [LPEI25K/ PMAA226K]16, gradually decrease in the swelling region, which corresponds to the NR results measured after drying the film show the small disintegration at the top surface with rearranged internal layers after the post-treatment for 10 min. Such slow disintegration kinetics of the multilayer films containing high MW PMAA chains are also hinted from the changes in dissipation energy (ΔDn) as a function of posttreatment time. The ΔDn indicates the loss of energy stored in a vibration cycle, which yields useful information on the changes in viscoelasticity as well as structural transformation of the adsorbed film. The dissipation energy typically increases when a viscoelastic film is strained during oscillation or when a liquid is trapped in or moves out of the films due to the deformation of the films.35−37 As shown in Figure 3B, the multilayer films with low MW PMAA chains, [LPEI25K/ PMAA15K]16 and [LPEI25K/PMAA32.4K]16, do not have significant increase in the dissipation energy, implying that no viscoelastic layer is placed on the quartz crystal sensor because the short PE chains losing electrostatic interactions are rapidly detached from top surfaces of the film into bulk liquid upon post-treatment. On the other hand, [LPEI25K/PMAA100K]16 and [LPEI25K/PMAA226K]16 show the significant increase in dissipation energy. The dramatic increase in the dissipation energy thus points to the increase in film softness as well as considerable hydration. Furthermore, the large increase in dissipation energy upon water uptake is the typical phenomenon of a structural transformation from a stiff and compactly bound structure into a dissipative nonrigid structure.39 The relation of the changes in dissipation energy to the changes in viscoelasticity and the structures of the films could be further detailed by analyzing the frequency dependence in the QCM-D responses of adsorbed films. The viscoelastic adsorbed layer typically shows the value of dissipation energy higher than 2.0 × 10−6 and different responses to overtones.40,49,50 In the present study, notable difference in the QCM-D responses with different overtones (n = 3, 5, 7) as well as the significant increase in the dissipation energy was observed during the slow disintegration process of weak PE multilayer films containing high MW PMAA chains, [LPEI25K/ PMAA100K]16 and [LPEI25K/PMAA226K]16, as compared to low MW PE counterparts. (see Figure 4 and Figure S1 in Supporting Information.) The frequency dependency in the overtones indicates that the multilayer films with high MW PEs, which quickly imbibe pH 2 water at the start of post-treatment, disintegrate very slowly from the top surfaces of the films, as confirmed by NR measurements, while the films gradually transform their initial compactly bound rigid structure into more viscoelastic and dissipative swollen state within the films. The fully swollen film finally reveals the disintegration characteristics with relatively

delayed disintegration from top film surface) in more controlled manner. Disintegration Kinetics and Corresponding Changes in Dissipation Energy of pH-Triggered Weak PE Multilayer Films with Different MWs. NR measurements provide useful information on the structural changes within multilayer films but usually take long time to obtain data, thus not feasible to measure short time release kinetics. As a complementary technique, QCM-D was employed to investigate the film disintegration kinetics and to obtain information on film properties and structural transformations during the posttreatment. The time for the complete disintegration of [LPEI25K/ PMAAMW]16 multilayer film masses dramatically changes ranging from 2 min to 5 days, depending on the MW of PMAA chains incorporated into the multilayer films (Figure 3A). An initial burst in the release profile was observed when a

Figure 3. QCM-D monitoring on the changes in (A) normalized frequency (%) and (B) dissipation energy of weak PE multilayer films, [LPEI25K/PMAAMW]16, with different MWs (solid line, 15K; dashes, 32.4K; dots, 100K; dash dots, 226K) of PMAA upon post-treatment of the multilayer films in pH 2.

low MW [LPEI25K/PMAA15K]16 film was subject to posttreatment with pH 2.0. The charge imbalance, triggered by the post-treatment at low pH, between adjacent PE layers (i.e., the increase in positive charges of LPEI chains at the expense of the decrease in negative charges of PMAA chains) causes the significant reduction in binding interactions, allowing the PE chains to disintegrate into bulk solution. However, the disintegration kinetics became slower as the MW of PE chains incorporated into the multilayer film was increased. In addition, we were able to monitor the considerable swelling of the films 4584

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Figure 4. Changes in frequency (blue curves) and dissipation energy (red curves) during the disintegration process of weak PE multilayer films with high MWs ((A) [LPEI25K/PMAA100K]16, (B) [LPEI25K/ PMAA226K]16) measured at three different overtones. The different curves within the same color correspond to third, fifth, and seventh overtones (n = 3 (solid line), 5 (dashes), and 7 (dots)) with driving frequencies of 15, 25, and 35 MHz, respectively.

Figure 5. Changes in the (A) frequency (Δf 3/3 (Hz)) and (B) dissipation energy of weak PE multilayer films triggered by sequential low pH values (i.e., pH 3.0, 2.5, 2.2, respectively for 10 min): [LPEI25K/PMAAMW]16 with low MW (15K, solid line) and high MW (226K, dotted line) of PMAA.

rapid increase in frequency and decrease in dissipation energy, independent to overtones, which corresponds to the disintegration of a soft and viscoelastic film from a quartz sensor to bulk liquid. The completely dissociated or detached multilayer films show the plateaus in both frequency and dissipation energy at the end of QCM-D measurements. Based on the QCM-D monitoring of frequency and dissipation energy as well as of its overtone dependency, we also note that the disintegration mode of the multilayer film is composed of the sequential events, instant water uptake, swelling, rearrangement and decomposition, as well as the kinetics is strongly dependent on the MW of polymer chains incorporated into the multilayer films. Swelling Kinetics of Model Multilayer Films with Different MWs as a Function of External Solution pH. The swelling behavior observed in the high MW [LPEI25K/ PMAA226K]16 film triggered at pH 2 was not detected in the case of the low MW [LPEI25K/PMAA15K]16 film presumably due to fast disintegration kinetics. To demonstrate the swelling behavior of low MW multilayer films before disintegration as well as to compare the swelling kinetics with high MW multilayer films, the responses in both frequency and dissipation of the multilayer films consisting of 15K and 226K PMAA chains were monitored by the sequential decrease in solution pH down to 2.2 instead of pH 2.0 (Figure 5). The [LPEI25K/PMAA15K]16 film shows the drastic changes in both frequency and dissipation energy even at pH value of 2.5 when it is compared with the [LPEI25K/PMAA226K]16 film

during the same post-treatment time of 10 min. These results clearly indicate that the multilayer film containing shorter MW PMAAs shows the rapid response in swelling at pH above 2.2 while the multilayer film containing higher MW PMAAs tends to maintain its original structure and rigidity for the same posttreatment condition. The slow kinetics in the sequential event involving swelling, rearrangements, and disintegration of high MW multilayer film could be originated from the more electrostatic binding units per PMAA chain as well as the fewer degrees of freedom of conformational rearrangements. The degree of ionization of PMMA chains incorporated within the multilayer films would significantly affect the film swelling/disintegration behavior as a function of pH. In this regard, the degrees of ionization (%) of PMAA chains in the [LPEI25K/PMAA15K or 226K]16 films were analyzed by FT-IR51 when those multilayer films were initially prepared at pH 5 and then subject to post-treatment at pH 2.2 (Figure 6). We noted that the initial [LPEI25K/PMAA15K]16 film containing about 24% ionized COO− groups in the PMAA layers represents only 5.3% of COO− groups after the post-treatment at pH 2.2. Similar trend was also found with the [LPEI25K/PMAA226K]16 film showing around 21% and 6.5% of degrees of ionization in the PMAA layers before and after the post-treatment at pH 2.2, respectively. This result points to the fact that, although the degree of ionization of the whole film is solely controlled by pH and independent of PE MW, the higher MW PMAA chains in the multilayer films could retain the swelling and, at the same time, retard the disintegration kinetics much longer because 4585

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there are more electrostatic binding units per chain when compared with low MW counterparts. Based on the combined analyses of NR, QCM-D, and FT-IR measurements, the disintegration behavior associated with the structural changes of weak PEs within multilayer films with different MWs is schematically summarized in Scheme 1. When the multilayer films are immersed into low pH water causing the deionization of PMAA chains along with excess positive charges of LPEI chains, the films tend to disintegrate into bulk solution in the entropy-driven manner. At the same time, the excess charges in LPEI layers could also cause the swelling of films due to charge repulsions, making both protonated PMAA and counter LPEI chains freely diffuse out into bulk solution from the top surfaces of multilayered films. In this disintegration process, the PMAA MW, which determines the number of binding units per chain, plays a major role in controlling the film disintegration kinetics. Furthermore, we also demonstrated that the rate determining step in the disintegration of weak PE multilayer films is the swelling process, which is again totally dependent on the MW of incorporated PE chains.

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CONCLUSION The MW-dependent swelling and disintegration behavior of model weak PE multilayer films, [LPEI25K/PMAAMW]16 (MW = 15K, 32.4K, 100K, and 226K), has been monitored by NR and QCM-D, yielding information on the nanoscale changes in internal structure and physical properties triggered by pH. The model films were prepared by the spin-assisted LbL deposition, which provides similar adsorption behavior of polymer chains regardless of PE MW and also offers flattened and smooth interfaces to enable precise monitoring of internal structural changes by NR. The [LPEI25K/PMAAMW]16 multilayer films containing high MW PMAA chains show the surface erosion while low MW PMAA multilayer films demonstrate the burst erosion when they are triggered by low pH, causing massive dissociation between oppositely charged layers. By the QCM-D measurements at different pH combined with FT-IR analysis, we demonstrated that the pH-triggered swelling and disintegra-

Figure 6. FT-IR spectra of PMAA chains of different MWs ((A) 15K and (B) 226K) incorporated within [LPEI25K/PMAAMW]16 multilayer films before (black open circle) and after (red open triangle) posttreatment, in the regions at 1701 and 1554 cm−1, associated with COOH and COO− groups, respectively.

Scheme 1. Schematic on the Disintegration Process of Spin-Assisted Weak PE Multilayer Films Containing Different MW PE Chains

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tion kinetics of the spin-assisted weak PE multilayer films are totally controlled by the MW of PE chains incorporated into multilayered films. Such MW-dependent disintegration kinetics originates from different numbers of binding sites per chain along with associated free energy penalty in conformational changes. This fundamental chain- as well as thin film structurallevel of understanding of the MW dependence on the pHtriggered disintegration kinetics of weak PE multilayer films could give insights into ways of developing functional controlled release platforms based on the LbL deposition without additional treatments or the insertion of other diffusion barrier molecules.

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ASSOCIATED CONTENT

S Supporting Information *

Characteristics of PEs used in the present study (Table S1) and the changes in frequency and energy dissipation during disintegration of [LPEI 25K /PMAA 15K ] 16 and [LPEI 25K / PMAA32.4K]16 measured at three different overtones (Figure S1). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*(K.C.) Telephone: +82-2-880-7431. Fax: +82-2-873-1548. Email: [email protected]. Present Address

∥ Department of Chemistry, Bogazici University, Bebek 34342, Istanbul, Turkey

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Research Foundation of Korea (NRF) funded by the Korea Ministry of Education, Science, and Technology (MEST) (The National Creative Research Initiative Program for “Intelligent Hybrids Research Center” (No. 2010-0018290), the Brain Korea 21 Program in SNU Chemical Engineering, and the WCU Program of Chemical Convergence for Energy and Environment (R31-10013)). Y. Jang also acknowledges the financial support from the Seoul Science Fellowship. We are very grateful to the NIST Center for Neutron Research (NCNR) for assigning enough beamtime to conduct NR experiments at NG7 beamline. In addition, we appreciate Dr. Jaseung Koo, Dr. Ki-Yeon Kim and Dr. Jung-Soo Lee for allowing us to use the REF-V beamline at the HANARO Center of Korea Atomic Energy Research Institute (KAERI).

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